A linear real-valued function of vectors such that . Vectors (i.e., contravariant vectors or "kets" ) and one-forms (i.e., covariant vectors or "bras" ) are dual to each other. Therefore

The operation of applying the one-form to a vector is called tensor contraction.

CITE THIS AS:

Weisstein, Eric W. "One-Form." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/One-Form.html